Optically encoded information, such as bar codes, have become quite common. A bar code symbol consists of a series of light and dark regions, typically in the form of rectangles. The widths of the dark regions, the bars, and/or the widths of the light spaces between the bars indicate the encoded information. A specified number and arrangement of these elements represent a character. Standardized encoding schemes specify the arrangements for each character, the acceptable widths and spacings of the elements, the number of characters a symbol may contain or whether symbol length is variable, etc. The known symbologies include, for example, UPC/EAN, Code 128, Codabar, and Interleaved 2 of 5.
Readers and scanning systems electro-optically decode each symbol to provide multiple alphanumerical characters that typically are descriptive of the article to which the symbol is attached or some characteristic thereof. Such characters are typically represented in digital form as an input to a data processing system for applications in point-of-sale processing, inventory control, and the like, Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,409,470 and 4,460,120, all of which have been assigned to Symbol Technologies, Inc., the assignee of this application.
To decode a bar code symbol and extract a legitimate message using such optical scanners, a bar code reader scans the symbol to produce an analog electrical signal representative of the scanned symbol. A variety of scanning devices are known. The scanner could be a wand type reader including an emitter and a detector fixedly mounted in the wand, in which case the user manually moves the wand across the symbol. Alternatively, an optical scanner scans a light beam such as a laser beam across the symbol, and a detector senses the light reflected from the symbol. In either case, the detector senses reflected light from a spot scanned across the symbol, and the detector provides the analog scan signal representing the encoded information.
A digitizer processes the analog signal to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars. The digitizer serves as an edge detector or wave shaper circuit, and the threshold value set by the digitizer determines what points of the analog signal represent bar edges. The threshold level effectively defines what portions of a signal the reader will recognize as a bar or a space.
The pulse signal from the digitizer is applied to a decoder. The decoder first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyzes the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard the scanned symbol conforms to. This recognition of the standard is typically referred to as autodiscrimination.
Different bar codes have different information densities and contain a different number of elements in a given area representing different amounts of encoded data. The denser the code, the smaller the elements and spacings. Printing of the denser symbols on an appropriate medium is exacting and thus is more expensive than printing low resolution symbols. The density of a bar code symbol can be expressed in terms of the minimum bar/space width called also "module size" or as a "spatial frequency" of the code, which is the inverse of twice the bar/space width.
A bar code reader typically will have a specified resolution, often expressed by the module size that is detectable by its effective sensing spot. For optical scanners, for example, the beam spot size could be larger than approximately the minimum width between regions of different light reflectivities, i.e., the bars and spaces of the symbol. The resolution of the reader is established by parameters of the emitter or the detector, by lenses or apertures associated with either the emitter or the detector by angle of beam inclination, by the threshold level of the digitizer, by programming in the decoder, or by a combination of two or more of these elements. In a laser beam scanner the effective sensing spot typically corresponds closely to the size of the beam at the point it impinges on the bar code. The photodetector will effectively average the light detected over the area of the sensing spot.
The region within which the bar code scanner is able to decode a bar code is called the effective working range of the scanner. Within this range, the spot size is such as to produce accurate readings of bar codes for a given bar code line density. The working range relates directly to the focal characteristics of the scanner components and to the module size of the bar code.
Typically, an optical scanner includes a light source, such as a gas laser or semiconductor laser, that generates the light beam. The use of semiconductor lasers as the light source in scanner systems is especially desirable because of their small size, low cost and low power requirements. The light beam is optically modified, typically by a lens, to form a beam spot of a certain size at a prescribed distance. The optical scanner further includes a scanning component and a photodetector. The scanning component may either sweep the beam spot across the symbol and trace a scan line across and past the symbol, or scan the field of view of the scanner, or do both. The photodetector has a field of view which extends across and slightly past the symbol and functions to detect light reflected from the symbol. The symbol electrical signal from the photodetector is converted into a pulse width modulated digital signal, then into a binary representation of the data encoded in the symbol, and then to the alphanumeric characters so represented, as discussed above.
Many known scanning systems collimate or focus the laser beam using a lens system to create a beam spot of a given diameter at a prescribed distance. The intensity of the laser beam at this point, in a plane normal to the beam (i.e., parallel to the symbol), is ordinarily characterized by a "Gaussian" distribution with a high central peak. The working range is defined as the region within which the intensely bright beam spot can decode the code after being scanned across the bar code symbol. But as the distance between the scanner and the symbol moves out of the working range of the scanner, which is typically only a few inches in length, the Gaussian distribution of the beam spot greatly widens, preventing accurate reading of a bar code. Present scanning systems, accordingly, must be positioned within a relatively narrow range of distances from a symbol in order to properly read the symbol.
U.S. Pat. No. 5,080,456 to Katz et al. proposed a bar code reader using a laser beam scanning system which has a greatly extended working range or depth of focus. In general, the scanning system included a laser source, an optical means for generating a diffraction pattern with an extended central beam spot of a prescribed diameter, and a scanning means for scanning the modified laser beam across a symbol. In the preferred embodiment, the laser source produced a regular "Gaussian type" optical beam which was modified by an optical element, such as an axicon. This optical element produces a beam which diffracts much less in the direction parallel to the bar code pattern. Specifically, an axicon will bend light from a point source on the optical axis so as to cross the axis along a continuous line of points along a substantial portion of the axis. The intensity and diameter of the beam spot created thereby will vary significantly along the distance of this line. An axicon also produces diffraction rings of light concentric with the central spot. A slit may be placed in the light path parallel to the scan line and perpendicular to the bars and spaces of the bar code symbol to be scanned. The slit removes the portions of diffraction which are perpendicular to the direction of scan, e.g., parallel to bars and spaces of the symbol. The slit, however, did not remove portions of the rings which were located in areas parallel to the scan or perpendicular to the bars and spaces. Although the Katz et al. system provided improvements over conventional lenses previously used in bar code scanners, further refinement of axicon design is necessary to optimize performance for bar code scanning applications. For example, Katz et al. did not consider how many diffraction rings should remain in the diffraction pattern for optimum detection within a maximal working range.
Additional problems relate to positioning the laser and lens within the scanner so as to set and maintain the. desired beam focusing. One approach has been to incorporate the laser source and lens into a module dimensioned to produce the requisite beam focusing. A laser diode and focusing module of this type will typically include a laser diode, at least one lens element for focusing light from the diode and means to fix the lens element at a desired distance from the laser diode so as to focus light from the diode at a point a predetermined distance in front of the module. Krichever et al., for example in their U.S. Pat. No. 4,923,281, teach telescoping two holding members of an emitting and focusing module against the force of a biasing spring positioned between the laser diode and the lens assembly to adjust the focusing of the light emitted by the module. One holding member is attached to the laser diode, and the other member holds the lens assembly for focusing the light from the laser diode. The second holder also provides an ellipsoidal aperture for the light passing through the lens. During actual focusing, the focusing module assembly is held in a jig which includes key or chuck elements to engage notches or keyways defining the orientation of the laser beam, lens and aperture as the two holding members are gradually telescoped together. As soon as the desired focus is achieved, the two holders are permanently fixed relative to one another by using adhesives such as glue or epoxy, or by fastening such as by staking, spot-welding, ultrasonic welding, or the like. Such focusing tends to require considerable labor by a skilled technician.
The focusing necessary for different scanning applications varies; a different focusing produces a different beam spot at different distances from the module. This produces a different working range and sensitivity for the scanner which must be chosen to correspond to the symbol density which the scanner will be expected to read and/or the preferred working range at which the scanner will be positioned. If a manufacturer produces scanners having a variety of working ranges and a variety of spot size sensitivities, the manufacturer must maintain an inventory of the above discussed laser diode and focusing modules preset to the focus appropriate for the particular scanner application the manufacturer expects each scanner product to service. Such an inventory is expensive to produce, particularly because of the labor intensive procedure for focusing each module.